1. Field of the Invention
The present invention relates to a toner for electrophotography and an image forming process, process cartridge and image forming apparatus using the toner.
2. Description of the Related Art
In electrophotography, a latent electrostatic image is formed on a photoconductor comprising a photoconductive substance, the latent electrostatic image is developed using a developer to form a toner image, the toner image is transferred to a transfer medium such as paper according to necessity, and the toner image is fixed thereon, for example, by heating and/or pressurizing to form a visible image. In electrophotography for forming color images, a document is exposed to light using a color separation filter, or an image is read from the document using a scanner and is written to a photoconductor using laser light to thereby form a latent electrostatic image of, for example, a yellow image area. The latent electrostatic image is then developed using a developer containing a yellow toner to form a yellow toner image, the yellow toner image is transferred to and fixed on a transfer medium such as paper, and a magenta toner image and a cyan toner image are then sequentially formed and fixed thereon to yield a color image.
Demands have been made on image formation using electrophotography to improve image quality, to perform image formation at higher speed, to provide more down-sized apparatus and to provide image formation with higher reliability. In particular in the formation of color images, colors must be reproduced in a wide range at a higher resolution. Thus, attempts have been made to decrease the softening points of color toners and to decrease the melt viscosity thereof to thereby improve color mixing in color toners of two or more colors.
However, toners having such thermal properties often invite offset to fixing rollers such as heating rollers upon heating and image-fixing procedure. To prevent offset, a technique of applying a releasing oil such as a silicone oil to the surface of the heating rollers, and a technique of adding a releasing agent to a toner have been proposed as disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 50-144446, 60-123856, 01-109360, 03-5764, 04-9067, 04-230769 and 08-50367.
However, the former technique requires an oil applicator, thus requires a complicated fixer and invites odor caused by the releasing oil upon image fixing. The latter technique invites filming on the surface of the photoconductor and is often deposited on the carrier in the developer or on a developing sleeve. In addition, a toner using a resin having a low melt viscosity cannot sufficiently prevent offset.
JP-A No. 11-282198 discloses a toner having a specific storage modulus and loss tangent (tan δ) and describes the use of crosslinked polymer as a binder resin. However, the present inventors have found that the single use of crosslinked polymer may not satisfactorily control the viscoelasticity of the toner, and process conditions such as the temperature in heating and image-fixing must be precisely controlled.
Objects and Advantages
Under these circumstances, an object of the present invention is to provide a toner which can be satisfactorily used in a low-temperature image-fixing system, has good offset resistance and is not deposited on image-fixing devices and images. Another object of the present invention is to provide a toner which can be satisfactorily transferred and removed from members in a process of cleaning and can produce sharp and clear visible images over a long period of time. Still another object of the present invention is to provide a process cartridge, image forming apparatus and image forming process using the toner.